Waveguide devices that involve transmission and manipulation of optical signals typically have a main, signal carrying region which has a higher index of refraction than that of the phases on either side of it. In such waveguide devices formed on soda-lime-silicate glass substrates, the region having a higher index of refraction is usually fabricated by exchanging for sodium, cations of higher molar refractivity than sodium as, for example, silver ions and large alkali ions such as potassium, rubidium, caesium, and thallium. In addition, in such waveguide devices, it is well known that there is an advantage in having: (1) fairly sharp boundaries between the higher index of refraction region and its neighbors; (2) minimal optical absorption in the signal-carrying, higher index of refraction region; and (3) a signal-carrying, higher index of refraction region which is buried deeply enough beneath the free surface of the glass substrate that scattering from surface irregularities and defects does not disturb the signal.
U.S. Pat. No. 3,880,630 issued on April 29, 1975 discloses a method for forming buried optical waveguides in glass substrates by means of a field-assisted ion-exchange process. In accordance with the teaching of the patent, buried waveguides are formed in a glass substrate by a first step of diffusing with the aid of an electric field ions having a large electronic polarizability per unit volume to produce a localized region of higher refractive index than the remainder of the material and by a second step of migrating the region of higher index of refraction to a desired depth by diffusing ions having a small electronic polarizability per volume again with the aid of an electric field. Further, the patent teaches that silver ions may be used as the first ions to be diffused into the dielectric substrate in the first step and that potassium ions may be used as the second ions to be diffused into the substrate in the second step. Still further, the patent teaches the use of masks to delineate the waveguides. Yet still further, the patent teaches the diffusion of the first and second ions from molten salt baths or from first and second layers of materials which are successively formed on the substrate. A further difficulty arises because the boundary between the exchanged layer and the unexchanged region does not remain planar. This occurs because the current distributes itself in a manner so as to diminish the resistance no matter what type of mask is used.
In addition, the patent teaches that the diffusions are to be carried out at relatively high temperatures just below the softening point of the substrate. In fact, the patent refers to 350.degree. C. as a relatively low temperature. This teaching to use relatively high temperatures causes a problem notwithstanding the fact that the high molar refractivity of the silver ion makes it an attractive candidate for use in forming waveguides. The problem arises because the negative free energy of formation of Ag.sub.2 O is a small value. As a result, some silver ions tend to be reduced to silver metal and this tendency increases with temperature. This is a problem because metallic silver atoms increase optical absorption and can prevent silver oxide rich layers in soda-lime-silicate glass from being useful signal carriers in many applications.